In the dry milling process used for manufacturing both food and fuel grades of ethanol from corn, a “beer mash” is made from which the ethanol is removed in a stripper column. The remaining mash is referred to as whole stillage or thick stillage in the fuel ethanol industries and thick slop in the beverage industry. The stillage which is typically in the range of 11% to 15% solids contains all of the other non-starch components of the corn kernel that pass through the process (germ, protein, gluten, hull & fiber etc.). Horizontal dewatering centrifuges are then typically used for removing a portion of the suspended solids from the whole stillage stream.
The centrifuges split the process stream into two fractions the first being a liquid stream called thin stillage and the second being the cake solids or distillers grains. The resulting solids or distillers grains, which typically contain about 65 to 85 percent water, are sent to a drying operation where the remaining water is removed by evaporation and the solids are dried to less than about 10 percent moisture. The dried solids, referred to as dry distiller grains (DDG's) are used as a nutrient source in the manufacture of certain animal feeds. In certain applications the material from the centrifuges may be hauled off site and disposed of by land application techniques or discarded in a landfill.
The liquid stream from the dewatering device is called centrate (thin stillage), which typically contains 6-10 percent solids by weight, with about 2 to 4% being suspended solids and about 4 to 6% being present as dissolved solids. The centrate or thin stillage from the centrifuge contains a number of valuable co-products some of which are soluble and some of which are suspended.
The thin stillage stream can be processed or used in a number of different operations within the plant. The decision as to how the stillage stream will be split and processed in a particular plant is based upon the economics of each available option. Typically a fraction of the centrate or thin stillage is sent back to the head of the plant as make-up water for the fermentation process, this stream is typically referred to as backset and may be as much as 50% of the thin stillage stream. The balance of the thin stillage stream is sent to an evaporation process where the water is removed and the dissolved and suspended solids are concentrated to a syrup with a solids content of 20 to 50 percent solids by weight. This material may then be blended with the distillers grains from the centrifuges or the dry distiller grains from the feed dryers to produce an animal feed at >88% solids commonly referred to as dry distillers grains with solubles (DDGS). The material can also bypass the drying operation and be supplied as a material referred to as wet feed at 30 to 40% solids.
The current standard in the dry grind ethanol industry is the use of high speed horizontal decanter type centrifuges for removing the suspended solids from the whole stillage or thick slop. The centrifuges are only effective in capturing a portion of the suspended solids in the whole stillage stream. Due to the high shear imparted in the unit a considerable portion of the smaller particles (fines) or the larger particles which are sheared can pass through the unit and are discharged in the centrate (thin stillage). A fraction of solids present in the thin stillage have a density very close to that of water and are extremely sensitive to shear making their removal in a centrifuge extremely difficult. We have observed that the fine suspended solids in the thin stillage (centrate) do not settle even when allowed to stand undisturbed for extended periods of time (24 to 48 hours or in some cases more). Another component of the whole stillage stream that is of some concern is the oil, which is carried through the process. The oil fraction is present in the whole stillage as the germ of the corn kernel is not removed or recovered in corn dry milling operations. The centrifuges used for processing stillage have been optimized for solids capture efficiency and as a result they only remove a portion of the oil present in the whole stillage stream.
The use of the processing aids described in this invention and the mechanical component as described in this invention have resulted in significant improvements in suspended solids capture efficiencies and the capture and removal of oils from the thin stillage, the backset and the syrup streams.
Various processing aids (flocculants, coagulants, agglomeration aids) have been evaluated in centrifuges in order to improve the discharged cake solids and reduce the solids in the centrate. Due to the physical characteristics of the centrifuges the improvements in cake solids or centrate quality as a result of the addition of anionic flocculants to the centrifuges was negligible.
Therefore, there is an ongoing need for improved solids/liquids separation technologies, dewatering and processing aids and the development of methods which improve the efficiency of solid-liquid separation in the whole stillage, thin stillage, backset and syrup streams, with concomitant reduction in the energy required to prepare the dry distiller grains and produce ethanol.